(1) Field of the Invention
This invention relates to the fabrication of a semiconductor device, and more specifically to semiconductor devices fabricated from silicon on insulator, (SOI), technology.
(2) Description of Prior Art
A critical objective of the semiconductor industry has been to continually increase the performance of silicon devices. The ability to reduce performance degrading, parasitic capacitances, resulting from diffusion to semiconductor substrate capacitances, has been accomplished via use of the silicon on insulator, (SOI), technology. The SOI technology consists of forming the desired devices in a layer of silicon, which overlies an insulator layer, which in turn overlies a semiconductor substrate. However the insulator layer in the SOI structure presents specific problems in achieving the desired device characteristics. For devices or transistors, fabricated using conventional, or without SOI technology, electrical contact to the semiconductor substrate is easily made. For example in metal oxide semiconductor field effect transistors, (MOSFET), devices, contact to a semiconductor substrate is made via use of the body node, or the undepleted material, directly under the gate electrode. However for SOI devices the body node is electrically floating, isolated from the substrate by the underlying insulator. This situation can result in undesirable drain currents, and lower threshold voltages. Other unwanted device characteristics, such as capacitive coupling between the body node and the source and drain regions, also adversely influence threshold voltages. Therefore efforts have been directed at developing processes that allow electrical contacts to obtained to the body node of the SOI layer, such as Houston, et al, in U.S. Pat. No. 5,185,280, or processes that allow electrical contact to the semiconductor substrate, such as Tashiro, in U.S. Pat. No. 5,241,211.
This invention will describe a process for fabricating MOSFET devices in an SOI layer, in which a back gate contact, or direct contact to the semiconductor substrate, is made without adding significant complexity to a process used to fabricate the desired MOSFET device.